Process for the conversion of cellulose in hydrated molten salts

ABSTRACT

A process for converting cellulose to glucose, said process comprising the steps of: providing a hydrated molten salt; contacting the hydrated molten salt with a cellulose-containing material to form dissolved glucose; removing the dissolved glucose from the hydrated molten salt.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for converting cellulose inhydrated molten salts.

2. Description of the Related Art

U.S. Pat. No. 1,943,176 to Graenacher discloses a process for dissolvingcellulose in N-ethylpyridinium in the presence of nitrogen-containingbases.

Zhu et al., Green Chem., 2006, 8, 325-327 discloses dissolution ofcellulose in a number of ionic liquids, in particular1-butyl-3-methylimidazolium chloride (BMIMCI) and1-allyl-3-methylimidazolium chloride (AMIMCI). Microwave heatingaccelerates the dissolution process. Cellulose can be regenerated fromthe ionic liquid by addition of water, ethanol, or acetone. The authorssuggest the use of ionic liquids for the fractionation oflignocellulosic materials and the preparation of cellulose derivativesand composites.

Swalotski et al. report on the use of ionic liquids, such as BMIMCI, forthe preparation of cellulose fibers. See U.S. Pat. No. 6,824,599 B2.

U.S. Pat. No. 4,999,149 to Chen discloses a process for high strengthcellulose fiber. Dissolving grade cellulose (i.e., cellulose that issubstantially lignin free) is dissolved in ZnCl₂ at elevatedtemperature. The cellulose/zinc chloride mixture is extruded into acoagulation medium.

The prior art processes use expensive materials for the ionic liquids.In addition, the disclosed processes do not convert cellulose other thanfrom one form of cellulose to another form of cellulose.

The present invention provides a process for dissolving low-gradecellulose in inexpensive ionic liquids. The present invention furtherprovides a process for purifying cellulose, and a method for convertingcellulose, in particular to cellulose fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reaction scheme for the conversion of cellulose.

FIG. 2 shows a schematic diagram of a reactor set-up for use in theprocess of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following is a description of certain embodiments of the invention,given by way of example only.

An important aspect of the process of the present invention is thediscovery that cellulose may be readily dissolved in hydrated moltensalts, which form an inexpensive class of Ionic Liquids. Suitable moltensalts include any salt with a melting point below 200° C., in particularhydrates of inorganic salts, the hydrates of ZnCl₂ being preferred,ZnCl₂.4H₂O being most preferred. An important aspect of the invention isthat the cellulose to be dissolved does not need to be pure. It maycontain significant amounts of lignin and/or hemicellulose. Accordingly,inexpensive sources of cellulose may be used.

Another important aspect of the process of the present invention is thediscovery that cellulose dissolved in a hydrated molten salt may beconverted to glucose simply by heating the solution. Preferably thesolution is heated to a temperature of at least 80° C., preferably to atemperature between 100° C. and 150° C. The glucose that is formed isdissolved in the hydrated molten salt.

When the solution is cooled off, the dissolved glucose converts back tocellulose. This property can be used to modify or regenerate cellulose,as has been described in the prior art. An important aspect of thepresent invention, however, is that the glucose that is formed isseparated from the hydrated molten salt. In one aspect the presentinvention provides a process for converting cellulose to glucose, whichcan be used as-is in, for example, the food industry, or be reactedfurther to desirable chemical compounds. An important aspect of theinvention is the further conversion of glucose to chemical compoundsthat are not soluble in the ionic liquid, which allows for their easyremoval from the reaction mixture.

FIG. 1 shows an overview of reaction paths available for the conversionof glucose to other valuable chemical compounds. For example, glucosemay be used as a starting material for the preparation of C₁₂ alkane,which is useful as a fuel for internal combustion engines, in particulardiesel engines, and as a heating fuel. Or glucose may be used to formmixtures of C₉/C₁₅ alkanes.

One way of removing glucose from the hydrated molten salt is byadsorbing the glucose onto a suitable adsorbent. Examples of suitableadsorbents include Sephadex.

Another way of removing glucose from the hydrated molten salt is byflushing the solution with a suitable solvent. The solvent should beimmiscible with the hydrated molten salt, and be a good solvent forglucose. Examples of suitable solvents include supercritical carbondioxide.

The various reaction steps shown in FIG. 1 can be carried out in theionic liquid medium. For clarification, the term “HMF” stands forhydroxymethyl furfural; the term HMTHF stands forhydroxymethyltetrahydrofuran. The various end products, such as C₁₂alkane and C₉/C₁₂ alkane mixture, are insoluble in the ionic liquid.These reaction products can be removed from the reaction mixture by wellknown standard techniques, such as phase separation and centrifugation.Thus, in one aspect the invention relates to a process comprising thesteps of:

-   -   a) dissolving cellulose in an ionic liquid;    -   b) converting the dissolved cellulose to glucose;    -   c) converting the glucose to a reaction product that is        insoluble in the ionic liquid.

In a preferred embodiment, the reaction product comprises alkanes, inparticular C₉, C₁₂ and/or C₁₅ alkanes.

The dissolution of cellulose in the ionic liquid and the conversion ofcellulose to glucose are enhanced by adding an acid to thecellulose/ionic liquid mixture. Examples of suitable acids includemineral acids, in particular HCl.

Step c) may comprise reactions selected from the group consisting ofdehydration, hydrogenation, aldol reactions, dehydrogenation, oxidation,alkylation, Guerbet reaction, and combinations thereof.

The term “hydrogenation” as used herein refers to mild hydrogenation,for example carried out by contacting the reactants with hydrogen in thepresence of a metal catalyst. A suitable metal catalyst is Raney nickel.Another suitable catalyst is Ru/C.

Aldol reactions are generally carried out in the presence of a base as acatalyst. Suitable bases include hydroxides, in particular alkali metalhydroxides.

Dehydration is catalyzed by Bronstedt acids, in particular inorganicacids, such as HCl.

Alkylation is catalyzed by Lewis or Bronstedt acids.

FIG. 2 shows an example of a reactor sequence suitable for a process ofthe present invention. In the first reactor a (ligno)cellulosic biomassis mixed with a molten salt. Optionally, an acid such as HCl is added toaccelerate the conversion of cellulose to glucose. A suitable reactiontemperature is in the range of 60 to 120° C. Optionally, hydrogen may beadded to the first reactor, as well as Raney nickel catalyst, tocommence the hydrogenation of glucose to polyols.

The second reactor is dedicated to the hydrogenation of glucose topolyols, using hydrogen gas in the presence of a hydrogenation catalyst,such as Raney nickel.

In the third reactor a Guerbet reaction takes place. In this reactionthe C₆ alcohol (e.g., sorbitol) resulting from the hydrogenation ofglucose is converted to a C₁₂ aldehyde. In a second step the C₁₂aldehyde is reduced to the corresponding C₁₂ alcohol.

Finally, in the fourth reactor, the C₁₂ alcohol is converted to C₁₂alkane. The C₁₂ alkane, being insoluble in the molten salt, is separatedfrom the molten salt. Molten salt, which contains the hydrogenationcatalyst, is recycled from the fourth reactor to the first reactor.

As is shown in FIG. 2, part of the polyol product produced in the secondreactor may be diverted to produce hydrogen. Hydrogen production may beaccomplished by subjecting the polyol (e.g., sorbitol) to a reformingreaction in the presence of a suitable catalyst. Ni/Sn is an example ofa suitable catalyst.

Lignin is not soluble in the molten salt medium. Due to its relativelylow density it floats to the surface of the first reactor, where it maybe removed by skimming. An alternative way of processing is usingconventional solid/liquid separation such as filtration.

Another aspect of the invention is the selective dissolution ofcellulose in the presence of lignin and/or hemicellulose.

Cellulose is nature's most abundant polymer. Important sources ofcellulose include straw, grasses, bagasse, wood, and the similarcellulose-containing forms of biomass material. Almost invariably,cellulose is present in these biomass materials in conjunction withhemicellulose and/or lignin. Processes exist for separating cellulosefrom hemicellulose and lignin. These processes tend to be expensive, andin many instances involve the use of corrosive or polluting chemicals.

Cotton is an example of a natural source of almost pure cellulose.Cotton is, as a consequence, a valuable material and economicallyunattractive as a raw material for cellulose-based processes. Algaecontain cellulose and little or no lignin. However, the cellulosecontent of algae is generally modest. As a result, algae are not a majorsource of cellulose for cellulose-based processes.

The present invention is based on the discovery that cellulose can beselectively liquefied in the presence of lignin. The liquefied cellulosecan subsequently be separated from the undissolved lignin. Accordingly,the present invention provides a process for producing a purified andliquefied cellulose from a lignocellulosic biomass material.

In a first step, the present process comprises providing a source oflignocellulose. As discussed hereinabove, nature provides numeroussources of lignocellulosic material. For the sake of conveniencereference will be made herein to wood as a lignocellulosic biomassmaterial, but it will be understood that any suitable source oflignocellulosic biomass material may be used.

Saw dust is an attractive starting material for the process of thepresent invention, because of its small particle size. If wood isavailable in the form of larger particles, such as wood chips, it isdesirable to reduce its particle size. Any available method for particlesize reduction is suitable for the purpose of the present invention.Particularly suitable methods include milling, grinding, and shredding.

The particles of lignocellulosic material are mixed with an ionicliquid. Preferred ionic liquids for use in the present process aremolten salts, in particular hydrates of inorganic salts. Particularlypreferred are the hydrates of zinc chloride, such as ZnCl2.4H20. Theinvention will be illustrated herein with reference to zinc chloride,but it will be understood that other ionic liquids may be used instead.

In order to dissolve the cellulose component of the lignocellulosicmaterial, it is necessary to heat the wood/zinc chloride mixture to atemperature in the range of from 40 to 200° C. An acid may be added inorder to increase the solubility of the cellulose component. Strongmineral acids are preferred for use herein, with HCl being particularlypreferred for reasons of cost, compatibility, and ease of removal.

In general, sources of lignocellulosic material comprise significantamounts of water. Even “dry” wood contains 5 wt % water or more. Thewater is freed up as the cellulose dissolves, and becomes part of themixture. In addition, the dissolution of cellulose in the ionic liquidinvolves the destruction of hydrogen bonds between cellulose polymerchains by abstraction of water molecules. In other words, thedissolution process itself produces water.

In order for the ionic liquid to retain its solvent strength it may benecessary to remove water from the mixture. For this reason it may bedesirable to operate at a temperature above 100° C., so that water isreadily removed by evaporation.

It has been found that higher temperatures and the presence of acidpromote the conversion of dissolved cellulose to glucose. Therefore, itmay be desirable to operate at a temperature of more than 80° C.,preferably more than 100° C., in the presence of an acid, if conversionof cellulose is desired. The amount of acid may be relative small,typically less than 1 wt %, generally in the range of from 0.1 to 0.8 wt%.

In a specific embodiment of the invention, the dissolved cellulose isused for the production of formed products, such as cellulose sheets,cellulose films, and cellulose fibers. For purposes of this embodimentit is important to avoid depolymerization of the cellulose, and theformation of glucose (which is the ultimate form of depolymerization) isundesirable in this context. In general, glucose formation can beavoided by operating at a temperature in the range of from 40 to 80° C.,preferably in the range of from 60 to 70° C.

The skilled person will appreciate that the dissolution rate decreasesas lower mixing temperatures are employed. If the mixing temperature isat the low end of the range, i.e., less than 60° C., it may be desirableto add acid to the mixture in order to increase the dissolution rate toan acceptable level. It has been found that the use of acid at theselower temperatures does not result in an unacceptably high conversion ofthe cellulose to glucose.

In order to remove water while operating at temperatures below 100° C.it may be desirable to operate under reduced pressure. In general, theoptimum pressure is near the saturated vapor pressure of water at theoperating temperature. This allows the water to readily evaporate,without causing violent boiling of the mixture. By way of example, thesaturated steam pressure at 65° C. is 0.25 bar. It is desirable tooperate the dissolution step at a reduced pressure of 0.25 bar if theselected dissolution temperature is 65° C.

Lignin, which is the other main component of lignocellulosic biomassmaterial, does not dissolve in the ionic liquid under the above-definedconditions. After the cellulose is fully dissolved the remaining ligninmay be removed by any known technique fro separating solid particlesfrom a liquid. Examples of suitable techniques include filtration,settling, centrifugation, and the like. Care should be taken that thetemperature of the mixture is maintained during the lignin-removal step,to avoid precipitation of previously dissolved cellulose (if thetemperature were allowed to drop), or depolymerization of the dissolvedcellulose (if the temperature were allowed to rise).

Lignin recovered from this separation step is a valuable raw materialfor the production of specialty chemicals.

In many cases the lignocellulosic biomass material further compriseshemicellulose. It is, in general, desirable to avoid contamination ofthe cellulose with hemicellulose if the cellulose is used for makingformed products, such as fibers or sheets. The presence of significantquantities of hemicellulose decreases the mechanical strength of thefiber or sheet.

Being less stable than cellulose, hemicellulose dissolves much fasterthan cellulose. Moreover, hemicellulose is converted to, among others,xylose under the conditions of the mixing step. Having a far greatersolubility than cellulose, xylose can be readily removed from themixture by any one of the techniques known to the skilled person. Anexample of a particularly suitable technique is solvent extraction with,for example, a polar solvent such as an alcohol or water. Alsoconventional solid/liquid separation techniques can be used such asfiltration.

The cellulose may be recovered from the solution by extruding thesolution into a coagulating medium. Examples of a suitable coagulatingmedium include the C₁ to C₈ alcohols and ketones, in particular thealcohols of the group of straight chain and branched chain C₁ to C₄alcohols, such as methanol, ethanol, propanol, and iso-propanol.Particularly suitable ketones include the C₃ to C₅ ketones such asacetone and methylethylketone (MEK).

The cellulose may be formed into fibers by extruding the solution intothe coagulating medium through an orifice having a plurality of smallextrusion holes, such as a spinneret. In general, the extruded fibersare not fully crystallized. Crystallization may be enhanced by one ormore of the following post-treatment steps.

The fibers are removed from the coagulating medium. Preferably, residualcoagulating medium is removed, for example by evaporation. The fibersmay be stretched by applying tension to the fibers. The stretchingaction acts to orient the polymer molecules in the fibers.

Subsequently the fibers may be submerged in a bath containing water.

Thus, the invention has been described by reference to certainembodiments discussed above. It will be recognized that theseembodiments are susceptible to various modifications and alternativeforms well known to those of skill in the art.

1. A process for converting cellulose to glucose, said processcomprising the steps of: a) providing a hydrated molten salt; b)contacting the hydrated molten salt with a cellulose-containing materialto form dissolved glucose; c) removing the dissolved glucose from thehydrated molten salt.
 2. The process of claim 1 wherein thecellulose-containing material is a cellulosic biomass.
 3. The process ofclaim 1 or 2 wherein step c) comprises selectively adsorbing celluloseto Sephadex.
 4. The process of claim 1 or 2 wherein step c) comprisesflushing the hydrated molten salt with a solvent.
 5. The process ofclaim 4 wherein the solvent is selected from the group consisting oforganic solvents, supercritical fluids, and mixtures thereof.
 6. Theprocess of claim 1 or 2 wherein the dissolved glucose is removed byconversion to a material that is poorly soluble in the hydrated moltensalt.
 7. The process of claim 6 wherein the glucose is converted to amaterial comprising an alcohol.
 8. The process of claim 7 wherein theglucose is converted to a material comprising an alcohol by a processcomprising mild hydrogenation.
 9. The process of claim 7 wherein theglucose is converted to a material comprising an alcohol by a processcomprising selective oxidation.
 10. The process of any one of claims 6-9wherein the conversion is carried out electrochemically.
 11. The processof any one of claims 6-9 wherein the conversion is carried out under theaddition of microwave energy.
 12. The process of any one of claims 6-9wherein the conversion is carried out under the addition of ultrasoundenergy.
 13. The process of any one of claims 6-9 wherein the conversionis catalyzed in the presence of a catalyst.
 14. The process of claim 13wherein the catalyst is a particulate solid material that is dispersedin the hydrated molten salt.
 15. The process of any one of claims 7-14comprising the further step of hydrogenating the alcohol to form analkane.
 16. The process of claim 15 wherein the alcohol is separatedfrom the hydrated molten salt prior to conversion to the alkane.
 17. Theprocess of claim 2 wherein the cellulosic biomass is substantiallylignin-free.
 18. The process of claim 17 wherein the substantiallylignin-free biomass is prepared by mild hydrothermal treatment oflignin-containing biomass.
 19. The process of any of claims 6-18 whereinthe conversion comprises a reaction is a micro channel reactor.
 20. Theprocess of claim 19 wherein the micro channel reactor is at leastpartially coated with a catalytic material.
 21. The process of claim 15or 16 wherein hydrogen used in the hydrogenation reaction is generatedby reforming cellulosic biomass.
 22. The process of claim 15 or 16wherein hydrogen used in the hydrogenation reaction is generated byreforming lignin.
 23. A process for selectively dissolving cellulose,said process comprising the steps of: a. Providing a source oflignocellulosic material; b. Contacting the lignocellulosic materialwith a molten salt under reaction conditions that minimize thedepolymerization of cellulose.
 24. The process of claim 23 wherein thelignocellulosic material further comprises hemicellulose.
 25. Theprocess of claim 24 wherein the lignocellulosic material is alignocellulosic biomass material.
 26. The process of any one of thepreceding claims wherein the hydrated molten salt is hydrated zincchloride.
 27. The process of any one of claims 23-26 wherein thereaction conditions comprise a temperature of from 40 to 80° C.
 28. Theprocess of claim 27 wherein the reaction conditions comprise thesubstantial absence of mineral acid.
 29. The process of any one ofclaims 23-28 whereby water is removed from the molten salt during stepb).
 30. The process of claim 29 whereby step b) is carried out underreduced pressure.
 31. The process of claim 30 whereby step b) is carriedout at a pressure which is at or near the saturated steam pressure ofthe reaction temperature.
 32. The process of claim 31 whereby step b) iscarried out at a temperature in the range of 60 to 70° C. and a pressurein the range of 0.20 to 0.30 bar.
 33. The process of any one of claims23-32 whereby lignin is removed from the molten salt during or afterstep b).
 34. The process of any one of claims 24-33 wherebyhemicellulose is converted to a reaction product comprising an alcohol.35. The process of claim 34 wherein the reaction product comprisesxylotol.
 36. The process of any one of claims 23-35 wherein the solutionobtained in step b) is extruded into a coagulating medium.
 37. Theprocess of claim 36 wherein the coagulating medium is selected from thegroup consisting of the C₁ to C₈ alcohols, the C₁ to C₈ ketones, andmixtures thereof.
 38. The process of claim 36 or 37 wherein thecellulose is extruded in the form of fibers.
 39. The process of claim 38wherein the cellulose fibers are subjected a post-treatment step forincreasing the crystallinity of the cellulose.
 40. The process of claim39 wherein the post-treatment step comprises stretching.